Embeddable corrosion rate meters for remote monitoring of structures susceptible to corrosion

ABSTRACT

An embeddable corrosion rate meter (ECRM) for detecting and measuring corrosion in metal and concrete structures is provided. The system comprises an electrochemical cell with at least one working electrode evenly separated from a counter electrode, wherein a separation distance between electrodes determines an electrolyte medium resistance and the electrolyte medium resistance is less than or equal to a polarization resistance. The system further includes a signal generator connected to a plurality of resistances for creating a plurality of current amplitudes for generating a current source; a first selector for applying a current through each of the plurality of resistances to the working electrode and counter electrode; a second selector for selecting a duration of a current pulse; a voltmeter/A-D converter having an input impedance &gt;10 9  ohms for measuring polarization of the working electrode; and an external reader-head with a data link and power link connected to a computing device for powering the system and collecting corrosion measurements data.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a national stage of international application no.PCT/US03/22606, filed Jul. 18, 2003, and claims priority to U.S.Provisional Application Nos. 60/396,694, filed on Jul. 18, 2002, and60/409,330, filed on Sep. 9, 2002, the contents of which areincorporated by reference herein.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to Embeddable Corrosion RateMeter (ECRM) instrumentation for remote monitoring of structuressusceptible to corrosion.

2. Description of the Related Art

The basic operating principles of the Embeddable Corrosion Rate Meter(ECRM) instrumentation works on the principles of a technique known aschronovoltammetry, i.e., Voltage-Time Response. However, recent changesto the instrumentation part of the ECRM provides the flexibility ofusing principles of yet another technique, known as alternating current(AC) impedance or Electrochemical Impedance Spectroscopy (EIS), toestimate corrosion rates. The ECRM instrumentation or sensor contains atest electrode that is perturbed or excited with one or more current (I)pulses. The time-dependent changes (response) in the electrochemicalpotential (Y) of the electrode are measured.

Alternatively, a set of constant potential pulses can be used as theperturbation signal to measure the resulting current transients(chronoamperommetry) and estimate the corrosion rates. Thisnotwithstanding, the rest of the application is directed tochronovoltammetry. The ECRM instruments or sensors are small, comparablein size to concrete aggregates and require very little electric power tooperate. The electronic circuit necessary for making the instrument isrelatively simple. The use of chronovoltammetry allows ECRM instrumentsto work in electrolytes, such as concrete, that are not good conductorsof electricity.

The (V/I) ratio, also known as the polarization resistance, R_(p), isinversely proportional to the corrosion rate. The conventional corrosionrate measurement techniques such as linear polarization and logarithmicpolarization also estimate R_(p) described in “Testing of Concrete inStructures”, Ed. J. H. Bungey and S. G. Millard, Blackie Academic &Professional, NY, Third Edition, 1996, p. 173. These techniques use adirect current (DC) or voltage source to perturb the electrode andmeasure the DC voltage or current response using relatively simpleelectronic circuitry. However, the conventional corrosion ratemeasurement techniques are not useful in measuring corrosion rates whenthe metal is in contact with mediums that are poor conductors ofelectricity. These techniques suffer from an error caused by theresistive drop, also known as “IR-Drop”, that occurs when the currentpasses through the resistive medium. Therefore, the use of linear andlogarithmic polarization techniques could result in erroneous estimationof corrosion rates.

There are also techniques based on alternating current (AC) principles.For example, AC impedance or electrochemical impedance spectroscopy(EIS) can measure R_(p) more accurately than the DC techniques, but itrequires complex electronic circuits. The chronovoltammetry-based ECRMemploys a relatively simple electronic circuit, overcomes the problem ofIR-Drop, can be designed to be small, and requires very little power tooperate. ECRM, which is an ideal corrosion rate meter, is embeddable inconcrete or soil to measure corrosion rates of steel reinforcing bars(rebars), pipelines, and other buried structures.

Similar to linear and logarithmic polarization, and EIS techniques, theECRM also uses principles of electrochemistry to measure corrosionrates. In essence, all electrochemical techniques apply a known voltageto the metal under test, and measure the resulting current flow acrossthe metal/electrolyte (concrete) interface. Alternatively, in somecases, the perturbing signal is a known current, and the resultingchange in the voltage across the metal/electrolyte interface ismeasured; the resistance across the electrode/electrolyte interface isthe polarization resistance, R_(p). The current-voltage relationshipprovides the rate of corrosion of the metal in the medium (concrete).

A major problem with most techniques is the electrical resistance of theconcrete: the current that flows through the concrete generates avoltage drop, V_(conc)=IR_(conc) (IR-Drop) across its resistance. Thus,the voltage applied or measured is V, which is the sum of IR_(conc) andIR_(p); IR_(conc)=V_(conc); IR_(p)=V_(p); and V=V_(conc)+V_(p). Inconcrete, R_(conc) can be much larger than R_(p), and unless thecorrection is made for the voltage drop, V_(conc), across R_(conc), thecorrosion rate will be grossly underestimated. Most electrochemicaltechniques suffer from this limitation, and some of them usechronovoltammetry for the IR-Drop correction. In other words, theycombine chronovoltammetry for IR-Drop correction with yet anothertechnique to measure the rate of corrosion. An obvious, practicallimitation is using at least two types of electronics andinstrumentation, one for IR-Drop correction, and another for corrosionrate measurement.

SUMMARY OF THE INVENTION

The inventive technique uses chronovoltammetry for both R_(conc)estimation and for corrosion rate estimation. Thus, the electroniccircuit used is the same, which is a particular advantage whiledesigning miniature, embeddable instruments. The instrument used toimplement the technique is about the size of a small pebble, normallyfound as aggregates in concrete, and is compatible with the device knownas the Smart Aggregate. The technique to implement corrosion ratemeasurements in, for example, concrete, using ECRM is described below.

Accordingly, in one embodiment of the present invention, an embeddablesystem for detecting and measuring corrosion in a structure susceptibleto corrosion is provided, said system including a plurality ofembeddable corrosion rate meters ECRM) for collecting corrosionmeasurements data and at least one computing device for analyzing saidcorrosion measurements, said system comprising:

at least one working electrode evenly separated from a counterelectrode, wherein a separation distance between said at least oneworking electrode and said counter electrode determines an electrolytemedium resistance, said electrolyte medium resistance is less than orequal to a polarization resistance;

a signal generator for generating a current source, said current sourceis connected to a plurality of resistances for creating a plurality ofcurrent amplitudes;

a first selector for applying current through each of said plurality ofresistances to said at least one working electrode and said counterelectrode, wherein said current is applied via a galvanostat; and,

an external reader-head with a data link and power link connected tosaid computing device for powering said ECRM and transferring corrosionmeasurements data via said data link.

A second embodiment of the present invention is a method for detectingand measuring corrosion in a structure susceptible to corrosion, saidcorrosion being detected by a plurality of embeddable corrosion ratemeters (ECRM) and analyzed by at least one computing device, said methodcomprising the steps of:

determining an electrolyte medium resistance using a separation distancebetween at least one working electrode and said counter electrode, saidat least one working electrode evenly separated from a counterelectrode, wherein a electrolyte medium resistance being less than orequal to a polarization resistance;

generating a current source connected to a plurality of resistances forcreating a plurality of current amplitudes;

applying a current from a first selector through each of said pluralityof resistances to said at least one working electrode and said counterelectrode, wherein said current is applied via a galvanostat;

selecting via a second selector, a duration of a current pulse;measuring polarization of said working electrode using a voltmeter/A-Dconverter, wherein said voltmeter has an input impedance greater than10⁹ ohms; and,

powering said system via a power link connected to an externalreader-head and collecting corrosion measurements data via a data linkconnected to said external reader-head, wherein said externalreader-head is connected to said computing device.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become more apparent from the following detaileddescription when taken in conjunction with the accompanying drawings inwhich:

FIG. 1A is a diagram illustrating a ring-disc sensor assembly includinga working electrode and a counter/reference electrode of the presentinvention;

FIG. 1B is a diagram illustrating a multiple sensor assembly includingfour working electrodes and two counter/reference electrode of thepresent invention;

FIG. 2 is an electrical diagram representing an Embeddable CorrosionRate Meter (ECRM) sensor circuit of the first embodiment;

FIG. 3A is a graph of a series of current pulses, I₁ to I₄, applied oneafter another, for each I, the pulses are applied twice, first for aduration of 1 ms, and next over 500 ms, the current pulse sequenceapplied to measure V_(conc) and V; and compute V_(p);

FIG. 3B is a voltage/time response, 1 ms in duration, used to measureV_(conc), the polarization is due to the electrolyte (concrete)resistance;

FIG. 3C is a voltage/time graph of the pulse, 500 ms in duration, usedto measure V, V_(p) is computed as (V−V_(conc)), and the plot of V_(p)/Iprovides, the polarization resistance, R_(p), which is inverselyproportional to the corrosion rate;

FIG. 4 is an electrical diagram representing the ECRM sensor circuit ofthe second embodiment, comprising a programmable electronic chip, agalvanostat, and an electrochemical cell of the present invention;

FIG. 5 is a graph of current I over time showing the current pulseoutput from the galvanostat of the present invention;

FIG. 6 is a graph of voltage V over time showing the voltage response ofthe electrochemical cell to the current input from the galvanostat ofthe present invention, shown in FIG. 5;

FIG. 7A is a graph of impedance of the Dummy Cell Solartron ECI TestModule 12861, with an equivalent R_(p) of 6,800 ohms; and,

FIG. 7B is a graph of impedance of the Electrochemical Cell with steelembedded in concrete.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

First, it should be pointed out that a rebar in concrete is alwayscorroding. The rate of corrosion is generally so small that it has noadverse effect on concrete. However, in addition to water, low pH andoxygen, when a corrosive agent such as chloride or salt gets to thesurface of the rebar, then the rate of corrosion will increase therebyresulting in damage to the structure. The inventive Embeddable CorrosionRate Meter (ECRM) technology described herein measures the corrosionrate directly, instead of inferring it from the concentrations ofvarious chemicals, i.e., corrosive agents, which initiate and sustaincorrosion.

The ECRM identifies the transition of the rate from being small andbelow the threshold to above the threshold of causing corrosion damage.It is important to identify the transition, since if the measurementindicates a rebar is already corroding above the critical limit, itcould be too late to prevent potential damage to concrete. The issue ofearly identification of the onset of the above-threshold corrosion rateis addressed herein by measuring the corrosion rates of a section ofrebar material placed vertically above the reinforcing rebar. Whencorrosive agents penetrate into concrete, they do so by starting fromthe surface of the concrete and moving inward. A rebar materialcorrosion rate sensor placed in the path of the corrosive agents at oneor more locations before the agents reach the reinforcing bars willserve the purpose of forewarning the impending corrosion to the rebars.

Advantages of Direct Measurement of Corrosion Rate

Before describing the design of the corrosion rate sensor, ECRM is firstcompared with a Conductivity Sensor in Smart Aggregate (SA), which alsosenses changes in the corrosive environment of concrete. The SA is fullydescribed in U.S. patent application Ser. No. 10/220,102, filed on Aug.27, 2002. The Conductivity Sensor is described in the U.S. patentapplication Ser. No. 10/344,000, filed on Feb. 6, 2003.

The intended purpose of the corrosion sensor is identical to the pH,CO₂, or conductivity sensor: to identify the incoming corrosive agents,e.g., acid rain, CO₂, and chloride, and indirectly determine whencorrosion will initiate in the rebars. The corrosion rate sensor has animportant advantage over the other sensors, namely, it more directlymeasures the corrosion rate. The rest of the sensors only measureparameters that are potentially related to corrosion, and may need inputabout a number of variables for accurate data interpretation. Aconductivity sensor used in monitoring chloride concentration, forexample, is an “indirect” sensor; relating conductivity data to chlorideconcentration depends upon a number other variables, including moisturecontent, temperature and the amount of chloride bound to the silicatesin concrete. On the other hand, the corrosion meter provides corrosionrate data at any temperature, moisture, or chemical properties of theelectrolyte medium like concrete. Moreover, a conductivity sensor willreport an increase in the conductivity, if sodium chloride is co-presentwith sodium sulfate. The later is present in brackish water andseawater, and unlike rainwater mixed with chloride, does not attack asteel rebar aggressively. A conductivity sensor cannot tell thedifference in conductivity caused by a benign salt dissolved in brackishwater over an aggressive salt dissolved in rainwater.

Furthermore, dry salt, without moisture, does not alter the electricalconductivity in concrete. Concrete, in general, is highlynon-conducting, and inclusion of salt should increase the conductivityof concrete. Laboratory measurements on concrete samples mixed withvarious amounts of sodium chloride, i.e., common salt, have shown thatthe conductivity does vary by more than one order of magnitude when theconcentration of the salt in concrete is increased to about 3,500 ppm,as described on page 170 of the above referenced “Testing of Concrete inStructures”. Recent measurements conducted at The Johns HopkinsUniversity Applied Physics Laboratory have raised some questions incorrelating corrosion rate with conductivity. There is, of course, noquestion that salt does increases conductivity in concrete. However, italso appears that water in wet concrete also increases the conductivity.While salt alone or water alone increase conductivity in concrete, thereis no way of telling the degree of contribution from salt and waterwithout a priori knowledge of the concentration of either the salt orthe water. That means, that a salt sensor, that specifically measuresthe concentration of chloride, and water and humidity sensor, toidentify the level of water, are necessary to determine the individualcontributions of salt and water to the overall measured conductivity.Even if these two sensors were used, they do not necessarily measure thetotal amount of salt or water present in the concrete, which dependsupon how the chloride ions and water molecules are bound to the otherchemical components in cement, therefore, may or may not contribute tothe measured conductivity.

Chloride tends to bind to the silicates, and water to calcium hydroxide,both major components of concrete, and the degree of binding may dependupon the age of concrete. Once bound, those ions and moleculescontribute toward conductivity to a lesser degree, and the degree ofcontribution from each is differently dependent upon temperature. Thus,predicting corrosion information from conductivity data could becomeintractable, unless pH, wetness, and temperature data are known. Theconductivity sensor may still be a useful indicator of potentialpresence of salt inside concrete, but its usefulness depends upondeveloping an accurate physicochemical model of conductivity vs.chemical/physical properties of concrete.

Corrosion rate data, unlike conductivity data, tell us if the corrosionrate has increased or not. Parameters that accentuate corrosion of steelin concrete are well documented in the literature, for example

(a) Proceedings of the symposium on Corrosion of Reinforcement inConcrete, 21–24 May 1990, Ed. C. L. Page, K. W. Treadaway and P. B.Bamforth, Published for Society for Chemical Industry by ElsevierApplied Science, NY, pp. 281–384;

(b) Properties of Concrete, Ed. A. M. Neville, John Wiley & Sons Inc.,NY, 1996;

(c) Life Prediction of Corrodible Structures, Ed. R. N. Parkins, NACEInternational, Houston, Tex. 1994, Section 4 Concrete, p. 52; and,

(d) C. E. Locke, “Corrosion of Steel in Portland Cement Concrete:Fundamental Studies,” in Corrosion Effect of Stray Current and theTechniques for Evaluating Corrosion of Rebars in Concrete, ASTM SpecialTechnical Publication 906, Ed. V. Chaker, ASTM, PA, 1984, p. 5.

Salt is just one of the agents that cause corrosion in a steel rebar.Water alone does not increase the rate of corrosion, it is the dissolvedoxygen in water that does. The rate of corrosion for a fixed amount ofsalt is dependent on the pH of concrete. Concrete, typically has a pH inthe range of about 13 to about 13.5 and at that pH steel rebars do notcorrode. Moreover, as long as the pH of concrete is within the range ofabout 13 to about 13.5, it does not corrode even in the presence ofsmall amount of chloride, e.g., about 350 ppm, an amount commonly foundin deiced surfaces of concrete bridge decks. The dependence of thecorrosion rate on the pH of the electrolyte (concrete) containingvarious amounts of chloride is described in the literature, for example,

(e) Lectures on Electrochemical Corrosion, Ed. M. Porbaix, Plenum Press,NY, 1973, p. 271; and,

(f) Properties of Concrete, Ed. A. M. Neville, John Wiley & Sons Inc.,NY, 1996, p. 566.

However, if the pH decreases from about 13 to about 12, then the rate ofcorrosion will be substantially higher in the presence of about 350 ppmchloride as compared to an absence of chloride, see references (e) and(f) above. If the pH decreases to about 11, the rate of corrosion causedby the same amount of chloride increases by several orders of magnitudeas compared to the rate at pH about 13. Acid rain, domestic sewage, andatmospheric CO₂ can and do decrease the pH of concrete from about 13.5to substantially lower values where the concrete is prone to corrosion,see page 560 of “Properties of Concrete, Ed. A. M. Neville, John Wiley &Sons Inc., NY, 1996”. The corrosion rate data obtained using the ECRMsensor do not require conductivity, pH, moisture, oxygen concentration,or chloride content data for interpreting the results. The corrosionrate is useful as such, without compensation for the chemical andphysical composition of concrete. Thus, the ECRM is superior toconductivity, humidity, oxygen, or pH sensors. ECRM also providesanswers to the only question that all other sensors are attempting toinfer: how long will it take before the steel reinforcing bars begin tocorrode at rates that are significant and detrimental to the structure?

The Corrosion Rate Sensor

FIGS. 1A and 1B illustrate two suggested corrosion rate sensor designs,a single and multiple sensor assembles; and it should be recognized thatother variations of the sensor designs are possible. In both designs,there is at least one working electrode (WE) 10 and onecounter/reference electrode (CRE) 12. The WE 10 is made from the samematerial as the subject that is undergoing corrosion; in the case ofconcrete, the WE 10 is made from the same steel alloy as the rebar usedin reinforcing the concrete. The CRE 12 is made from a non-corroding,inert material such as, for example, nickel, mixed-metal oxide, e.g.,titanium oxide+ruthenium oxide, graphite or ‘dimensionally stable’palladium-coated titanium. If necessary, although less desirable, thesame material as the WE 10, i.e., steel, can be used as the CRE 12. Itis quite important to keep the area of the WE 10 small; CRE 12 should beat least 25× larger than WE 10. In all the designs, the separationdistance between the perimeter edges of the WE 10 and the CRE 12 can be0.1 cm or more, limited only by the distance allowed by the innerdiameter of the external CRE 12, and the diameter of the central CRE 12(FIG. 1 b).

For example, in FIG. 1A WE 10 is about 0.3 cm in diameter and CRE 12 isabout 2.0 cm in outside diameter (OD) and about 1.3 cm in insidediameter (ID), and in FIG. 1B each WE 10 is about 0.2 cm in diameter,the external CRE 12 region is about 2.0 cm OD and about 1.6 cm ID andthe central CRE 12 region is about 0.8 cm in diameter. In the designshown in FIG. 1B, each WE 10 is interrogated separately for corrosionrate. The ratio between the surface areas of the WE 10 and CRE 12 shouldbe at least about 1:25, the CRE 12 being about 25-times larger than theWE 10. If it turns out that the rebar material should be used for theCRE 12, then the area of WE 10 and CRE 12 could be equal, and the sum ofthe areas of the two electrodes should be accounted for when estimatingthe corrosion rate. In other designs, the CRE 12 can have a cylindricalprojection of several millimeters above the top surface of the sensor;in these designs, the cylindrical surface will add to the total area ofthe CRE 12.

The separation distance (d) between the edges of the WE and the CREdetermines the electrolyte medium, e.g., concrete, resistance, R_(conc).The resistance associated with the corrosion rate, R_(p), also known asthe polarization resistance, is independent of d. Since the primaryobjective of the corrosion rate sensor is to measure R_(p), it isimportant to keep the R_(conc)≦R_(p); it also provides criticaladvantage from instrumentation perspective of keeping most of thedynamic range of the measurement devices to measure R_(p) rather thanR_(conc). The sensor designs described in FIG. 1A take the R_(conc) vs.R_(p) relationship into account, and provide the maximum advantage tomake accurate measurements of R_(p).

A ring-disc design of the corrosion sensor is described in FIG. 1A. Thesuggested shape for the WE 10 is a disc with a surface area of about0.071 cm² or about 0.3 cm in diameter. The CRE is a ring with a flatsurface; the OD and the ID are about 2.0 and about 1.3 cm, respectively.The surface area of the CRE 12 is about 1.8 cm². The WE 10 and CRE 12are positioned concentrically; it allows a uniform current densitydistribution over the two electrodes during rate measurements. The discshaped WE 10 and the ring shaped CRE are laid on an inert support,exposing only the flat top surfaces. The exposed front surfaces of theWE and CRE are in contact with the medium, i.e., concrete. The twoelectrodes are connected from their masked backside to the inputterminals of the Corrosion Rate Meter, which will be described belowwith reference to FIG. 2 and FIG. 4.

FIG. 1B illustrates an alternate WE/CRE design for the sensor. It allowsfor measuring of the distribution of corrosion rate over the area of thesensor. It consists of a CRE 12 split into a ring and a disc. The ringhas an OD and an ID of about 2.0 and about 1.6 cm, respectively; and thedisc is about 0.8 cm in diameter. The ring and the disc are electricallyconnected to each other and together form the CRE 12. The total surfacearea of the CRE is about 1.63 cm². There are fourindividually-addressable independent disc-shaped WEs 10, located about90° away from each other and placed between the ring and the disc areasof the CRE 12. WEs 10 are not electrically connected to each other. EachWE 10 disc is about 0.2-cm in diameter, has a surface area of about 0.03cm². The minimum separation distance (d) between the each WE and thering-disc CRE is about 0.1 cm. All the electrodes are set parallel tothe holder 14. The holder and the supports are made from an inertmaterial. The holder also houses the corrosion rate meter describedbelow with reference to FIG. 2 and FIG. 4.

The Corrosion Rate Meter

FIG. 2 shows the ECRM, which adheres to principles of current-pulsechronovoltammtery. The ECRM sensor circuit 20 includes a current sourcegenerated by a power source 28 connected through different resistancesR₁ 26 to R_(n) 24. Selecting the resistance using the Relay 2 (notshown) determines the amplitude of the current. Relay 1 16 selects theduration of the current pulse. The current is applied between the WE 10and CRE 12 of the ECRM through the galvanostat 22, and the polarizationof the WE 10 is measured by the voltmeter/A-D converter 18. Thevoltmeter 18 has an input impedance greater than 10⁹ ohms. A Galvanostatis an electronic instrument that controls the current through anelectrochemical cell at a preset value. An external reader-head with adata link and power link 32 powers the system 20, reads and transfer thedata via the data link 36 of the ECRM 20 to the computer 30.

The ECRM technique may be viewed as an extension of the conductivitymeter (CM) suggested in the patent application for the Smart Aggregate(SA). Both of them use a galvanostat 22 to inject a constant currentbetween two electrodes WE 10 and CRE 12 over a fixed period, and measurethe resulting difference in the voltage between them. In the CM, the twoelectrodes 10 and 12 have identical dimensions, and are made frommaterial such as gold or platinum-coated gold. In the ECRM case, the WE10 is made from the corroding metal, and has a different dimension fromthe CRE 12. However, there are several major differences between CM andECRM 20. Unlike the CM, which uses a current pulse with fixed amplitude,the ECRM 20 employs current pulses with several different amplitudes.Furthermore, at each current amplitude, two pulses with two differentdurations are applied. The first pulse is about 1 ms long, and thesecond pulse is about 500 ms in duration. The voltage differencesbetween the WE 10 and CRE 12 are measured before the pulse is applied,and the end of the pulse, e.g., at about 1 ms and about 500 ms. Theschematic of the current pulses and typical current-time (I-t) responsesat about 1 ms and about 500 ms are shown in FIG. 3.

The ECRM operation is performed by disconnecting the current sourcegalvanostat 22 from WE 10 and CRE 12 and measuring the voltagedifference between WE 10 and CRE 12. This is an open circuit voltage(OCV) between the two electrodes.

The measurement is performed as follows:

-   1. Set j=0, where j is an exemplary value from 0 to 4.-   2. Increment j and set current pulse amplitude to I_(j), the    suggested amplitudes for the current pulses are in the ±0.1 to ±10    μA range;-   3. Start the 1 ms current pulse at set amplitude and measure the    voltage difference between WE 10 and CRE 12. This is the 1 ms closed    circuit voltage (CCV_(@1 ms)) between the two electrodes for the    current pulse at set amplitude I_(j);-   4. Start the 500 ms current pulse with set amplitude and measure the    voltage difference between WE 10 and CRE 12. This is the 500 ms    closed circuit voltage (CCV_(@500 ms)) between the two electrodes    for the current pulse at set amplitude j. The difference between    CCV_(@1ms) and CCV_(@500 ms)provides (V_(p))_(j);-   5. Repeat Steps 2–4 for current amplitude values of I₂ through I₄,    as well as at −I₁, −I₂, −I₃ , and −I₄ and estimate the value of    (V_(p))_(j);-   6. Make a graphical plot of I vs. V_(p), with OCV as the origin.    Estimate the slope of the plot of I vs. V_(p). The slope provides    the value of the polarization resistance, R_(p), which is inversely    proportional to the corrosion rate;-   7. In a variation of this approach, j can be varied from ±1 to any    other number.

The voltage difference for the 1-ms pulse represents the voltage dropacross the electrolyte (concrete) resistance, and represented asV_(conc). The voltage difference for the 500 ms pulse represents thevoltage drop across the WE/electrolyte (concrete) interfacial resistanceplus the voltage drop across the concrete resistance, and represented asV_(p). V_(p)=(V−V_(conc)), and it represents the polarization at theWE/electrolyte (concrete) interface. The steps described above allow oneto measure V_(p) for a set of current pulses (I), typically in the rangeof ±0.1 to ±10 microamperes (μA). For the 0.071 cm² steel WE inconcrete, and for I in the range of ±0.1 to ±10 μA, the anticipatedrange of V_(p) is 0 to ±10 mV. The slope of the plot of I vs. V_(p),provides polarization resistance R_(p), which is inversely proportionalto the corrosion rate, see D. C. Silverman, “Practical CorrosionPrediction Using Electrochemical Techniques” in Uhlig's CorrosionHandbook, Ed. R. W. Reive, Electrochemical Society Series, SecondEdition, John Wiley & Sons, NY, 2000, p. 1179, and Peabody's Control ofPipeline Corrosion, Second Edition, Ed. R. L. Bianchetti. NACEInternational, TX, 2001, p. 307.

The electrical circuit for the ECRM is organized in such a way that itproduces at least two, more usefully four different current pulses ineach direction, current flowing from WE to CRE, and vice versa. Thesequence for the current pulses can be generated using software or ananalog circuit. Thus, the schematic in FIG. 3 can be rearranged inseveral ways to generate the current pulses. The voltage measurementcircuit in the schematic has an input impedance that is greater 10⁹ohms. Similar to the CM in SA, the ECRM can be powered by an externalpower source through inductive coupling. The data from the ECRM can betransmitted through RF to an external receiver. Thus, the ECRM can beimmersed or buried in a medium, and it can remain passive, until it isactivated by an external stimulus.

Application

The ECRM has a large number of applications, especially to measurecorrosion rates in buried structures, for example, a metal, e.g., iron,steels, e.g., carbon steel, stainless steel, super alloy steels, etc.,copper, zinc, aluminum, titanium, and alloys and combinations thereof,in concrete and pipelines in soil, or immersed structures such as metaltanks filled with and immersed in chemicals such as, for example, acids,bases or an alkali medium, e.g., potassium hydroxide, sodium hydroxideand mixtures thereof. Most specifically, it can be kept in the upstreamof incoming corrosive agents well ahead of where the agents have achance to reach the structure that can be damaged by corrosion. Anychange, i.e., increase, in the corrosion rate of the sensor willindicate impending corrosion damage to the structure on the downstreamside. By locating one or more sensors above the rebars, the corrosiveeffect of incoming corrosive agents, such as chloride or change in pHcaused by CO₂ or acid rain, can be inferred before the corrosivechemical flux reaches the rebar. Thus, the ECRM corrosion sensormeasures the impact of those changes, and provides an advanced warningbefore the rebars ever experience corrosion.

The ECRM is different from the chloride, conductivity, temperature, orpH sensors: the ECRM provides direct information on the impendingcorrosion to the structure. The rest of the sensors, provide indirectinformation on the impending corrosion to the structure. Thus, if theobjective is to obtain a direct estimation of corrosion, then the ECRMis better than the rest of the sensors.

Pulse-Modulated Perturbation Signals

In an alternative embodiment of the present invention, the miniatureinstrumentation is developed for the purpose of generating a train ofpulses and applying them to the ECRM sensor. Typical shapes of theapplied Current-Time (I-t) pulse and the Voltage-Time (V-t) response arealso described herein. Experimental results, estimates of corrosionrates obtained using a corrosion sensor embedded in concrete, andvalidation of the performance of the newly developed miniatureinstrument by an independent, commercial, bench-top instrument aredescribed below. Those results show that the ECRM sensor, which includesthe miniaturized chronovoltammetry/AC impedance instrumentation,performs well in concrete.

This embodiment describes pulse-modulated signal source that synthesizesan I-t signal equivalent of a sum of several sine wave-signals, which isquite unlike the above-described embodiment that synthesized and appliedseveral square-shaped current pulses, but one pulse at a time.

The I-t perturbation signal (drive) is a sum of several sine wavesignals at different frequencies, typically in the range of 0.05 to 1000Hz. The resulting drive amplitude at each frequency is less than 1microampere; the sine wave at any particular frequency is slightlyphase-shifted from the others, such that the amplitude due to the sum ofall the frequencies does not exceed 2 microampere current at any time.The I-t drive is applied to the electrode galvanodynamically, therequired instrumentation for the galvanostat is also a part of the ECRMsensor. Applying a 2 microampere current on the 0.071 cm² area electrodein the sensor is equivalent to applying about 28 microampere current ona 1 cm² area electrode. If the area of the test electrode is designed tobe different from 0.071 cm², then the amplitude of the I-t signal shouldbe changed proportionately. The area change in the test electrode WE 10may warrant an area change in the counter electrode CRE 12. The totalduration of the I-t perturbation signal lasts for about 40 to about 80seconds, so that the signal at every frequency completes several cyclesduring the course of the test, which helps to improve the signal/noiseratio in the V-t response. The typical V-t response is in the 0.1- to100-mV range. Furthermore, one could apply the I-t perturbation signalsof amplitudes that are different from the suggested 28 microampere/cm²,but with caution. Applying a current of amplitude less than 56microampere/cm² may not alter the outcome (estimated corrosion rate).Currents with amplitudes above 56 microampere/cm² may adversely affectthe corrosion rate estimates. Currents below 14 microampere/cm² willcause the response signal (V-t) to be too small, affecting the accuracyof voltage measurement.

The I-t perturbation drive and the V-t response are analyzed usingcomputer software, which employs a conventional mathematical techniqueknown as Fourier analysis. The technique separates the individualfrequency components from the mixture, and estimates the time-delaybetween the applied signal and the response at each frequency. Thevector ratio for each voltage/current sine-wave pulse from the knowntime-sequence of the applied I-t pulse, and the V-t response is thenestimated. The vector ratio is generally referred to as impedance, fromwhich the rate of corrosion can be derived. The impedance is related tothe corrosion rate through a quantity known as polarization resistance,R_(p).

Note that the procedure described in the previous embodiment shown inFIG. 3 obtains R_(p) from a plot of I vs. V. The approach of the presentembodiment obtains R_(p) from the so-called impedance plot. In bothembodiments, R_(p) provides an estimate of the corrosion rate.

ECRM Instrumentation

FIG. 4 shows the electrical diagram of the ECRM sensor 40. The sensorincludes a miniaturized instrumentation for generating the I-t drivecurrent using a programmable electronic Chip 42 such as, for example,MicroStamp11™, and a galvanostat 22, that is applied to the test metalembedded or immersed in the Electrochemical Cell 44. Microstamp11 is oneof the world's smallest 68HC11 module manufactured by Technological Arts(819-B Yonge St., Toronto, Ontario, Canada M4W 2G9). The I-t pulseoutput of the Galvanostat 22 is shown in FIG. 5.

The Cell 44 contains the test metal WE 10, whose corrosion rate is beingmeasured, and a non-corroding counter/reference electrode (CRE) 12,either embedded or immersed in the corrosive medium. The purpose of theGalvanostat 22 is to generate a current pulse (I), which perturbs the WE10 in the Cell 44 and generates a voltage (V) response. The shape of theI-t signal is programmed into the Chip 42, which drives the Galvanostat22.

A prototype of the ECRM was tested in the laboratory, in which the testmetal, WE 10 was steel, cut in the form of a disc of small area 0.071cm², and the CRE 12 was a large area of 10 cm² stainless steel in theform a of a cylinder, placed concentric to the WE 10. The WE 10 and theCRE 12 were embedded in concrete, and together they formed the Cell. TheChip 42 was programmed to output ten different sine wave voltage signals(V_(chip)), simultaneously, in the 0.1 to 100 Hz frequency range. TheV_(chip) was the input for the Galvanostat 22, which outputted a currentsignal I of amplitude equal to (V_(chip)/R₁). The V_(chip) and the R₁values are so selected to keep I at about 1 microampere. The Chip wasprogrammed so that V_(chip), and therefore I, would last for 50 secondsas the input to the Electrochemical Cell 44. During the50-second-period, the lowest frequency component set at 0.1 Hz repeateditself five times, and the highest frequency component set at 100 Hzrepeated itself 5,000 times. The flow of I through the Cell 44 generateda voltage, V across the Cell 44. Due to the large difference in the areabetween the CRE 12 and WE 10 (see FIG. 1), most of V was assumed to haveoccurred across the interface between WE 10 and concrete, with the restacross the resistance of the concrete, and little across the CRE12-concrete interface.

The shape of a typical I and V is shown in FIGS. 5 and 6. The currentpulse output from the Galvanostat shown in FIG. 5, is the input signalto the electrochemical cell 44 (FIG. 4). In the illustrated example, thecurrent signal started at 8.5 second and ended at 58.5 second, for atotal period of 50 seconds. The start time of the current signal can beset to any value, but the start time should be accurately recorded forthe purpose of analysis.

FIG. 6 shows the V-t response by the electrochemical Cell 44, to theapplied I-t perturbation. The I-t pulse is the resultant of the sum ofseveral sine wave signals, each at a different frequency, and the signalat each frequency lasts for the entire duration of the sum. Theinstrument 42 (FIG. 4) performs the simple process of generating the‘summed up’ signal. Note that the summed up signal can be deconvoluted,and its individual component separated either by using other instrumentssuch as a Fast Fourier Transform (FFT) analyzer, or through mathematicaltechniques such as Fourier, Hadamand or Z transformation, all fairlywell known techniques. The discussion above is based on the mathematicaltechnique of Fourier transformation. The transformation allowsestimation of the impedance, the vector ratio of I and V, at eachfrequency.

An onboard FFT analyzer may be built within the ECRM sensor 40. Insteadof collecting a large time-array of I data and V data, and thenanalyzing it externally, a miniature FFT analyzer can be incorporatedon-board of the ECRM sensor 40. The FFT analyzer will take the I-tsignal shown in FIG. 5 and the V-t response shown in FIG. 6 through twoof its input channels, analyze them, and output the impedance data ateach frequency. Such an approach has two advantages. First, it willeliminate the need for transmitting large amounts of I-t and V-t dataout of ECRM for analysis purpose; typically, one set of I-t and V-t datamay contain anywhere between 1,000 to 100,000 data points. If an FFTanalyzer is on-board the ECRM sensor, then the number of pointstransmitted out of it is only about 40, assuming the total number offrequencies in the signal are twenty. Second, the presence of onboardFFT analyzer will eliminate the need for computer software to conductthe Fourier transformation, thus reducing the complexity associated withdata analysis. As a part of the ECRM technology development, a miniatureFFT analyzer is being incorporated inside the ECRM sensor 40. The testresults from the ECRM sensor 40 that includes the FFT analyzer willbecome available in the near future.

FIGS. 7A and 7B show the impedance values obtained for two differentelectrochemical Cells 44. Note that the impedance representation in thefigure belongs to a category known as complex plane plot; the propertyof the interface between the electrode and the concrete is such, that ithas the capacity to shift the applied current pulse and the resultingvoltage response in the time domain. Due to this shift, the vector ratiobetween the voltage and the current should be calculated using complexalgebra, an area of mathematics known to most engineers. Note that theshift depends upon the frequency of the applied signal, and the propertyof the electrode/concrete interface. However, in most cases, theimpedance plot will have a shape that is reminiscent of a partial circle(quarter circle to semicircle). The diameter of the circle is thepolarization resistance, R_(p), which is related to the corrosion rate.The inset in FIG. 7 a shows the electrical components in Module 12861.Note that the two instruments have measured and identified 6,800 ohms asthe difference in the Z_(real) values between 0.1 Hz and 100 Hz.Similarly, the Z_(real) value at 100 Hz is 2,800 ohm, which is the sumof 1,000 ohm and 1,800 ohm. In a real corrosion cell, the Z_(real) valueat the high-frequency limit (100 Hz) would represent the electrolyteresistance (or the concrete resistance R_(conc)), and the difference inthe Z_(real) values between the low- and high-frequency limits (0.1 Hzand 100 Hz) would represent the R_(p).

ECRM Data Analysis and System Validation

FIGS. 7A and 7B illustrate two sets of data collected on two types ofelectrochemical cells 44 (FIG. 4). One set was obtained on a “dummy”electrochemical cell with resistors and capacitors (Solatron Dummy Cell,ECI Test Module 12861) mimicking a metal/electrolyte (or steel/concrete)interface, and on a real Cell with steel embedded in concrete. Withineach set of data, there are two subsets, one collected using theminiature instrument 40 (FIG. 4), and another a bench-top, commercialimpedance-measuring instrument (Solatron SI 1287/1250). The commercialinstrument is used by the corrosion industry and was used herein as astandard to verify the newly designed miniature ECRM sensor 40 (FIG. 4).

The objective of experimenting with a dummy electrochemical cell is toprovide an initial comparison between the two approaches withoutintroducing the uncertainties that is sometimes present in a realcorrosion cell. In its simplest form, the current-voltage behavior ofthe interface between the electrode and the concrete is similar to thatof a capacitor connected in parallel to one resistor, and in series witha second resistor (see inset in FIG. 7B). The resistor in parallel withthe capacitor is the polarization resistance (R_(p)), which is relatedto the corrosion rate. The second resistor, R_(conc) is equivalent tothe electrical resistance of the concrete. The capacitor, C_(dl)indicates the state of the capacitance at the interface, therefore, theamount of corrosive chemicals (for example, chloride ions) adsorbed onthe electrode surface. The main objective of the ECRM is to estimateR_(p), thus the corrosion rate, although the response by the sensor tothe applied signal will be affected by R_(conc) and C_(dl). In practice,no a priori information will be available on R_(p), R_(conc) or C_(dl).Therefore, estimating those values using any instrument is questionable,unless the instrument is first calibrated with an electrical circuitwith known circuit elements. The dummy electrochemical cell, with knownelements of R_(p), R_(conc) and C_(dl), served the purpose ofcalibrating the commercial bench-top unit, which in turn was used tovalidate the ECRM sensor. Without independent validation such as these,the usefulness of a newly developed corrosion rate meter cannot beestablished.

The two subsets of impedance data in FIG. 7 a is the impedance due tothe Solartron Dummy Cell, ECI Test Module 12861, obtained usingSolartron SI 1287/1250 impedance measuring instrument, and the newlydesigned miniature instrument in FIG. 4. The circuit diagram of theModule 12861 is shown in the inset in FIG. 7 a. In FIG. 7 a, the twoimpedance plots have shapes that are fairy close to a semicircle,although there is more scatter in the data for the miniature instrument;the error in R_(p) caused by the scatter is less than 10%, which iswithin acceptable experimental limits. The dummy cell had an equivalentof 6,800 ohms for the R_(p). The diameter of the two semicircles matchedthe 6,800 ohm, suggesting that the commercial unit and the ECRM sensordo function properly.

The two subsets of data in FIG. 7B were obtained on a realelectrochemical cell, also using the Solartron SI 1287/1250 and the ECRMsensor. The match between the two subsets is reasonable within thefrequency range (0.1 Hz to 100 Hz) of the experiment. At the higherlimit of the frequency (100 Hz), the imaginary component of theimpedance is close to zero, and the real component of the impedanceprovides an estimate of the concrete resistance, R_(conc). For steel inchloride-contaminated concrete, at all frequencies above 100 Hz, thereal and imaginary components of the impedance converge at R_(conc). Atfrequencies below 0.1 Hz (data not shown), the impedance is limited bymass transfer of the reactant (in this case, oxygen, which is theoxidizer). In this region, the Z_(real) vs. Z_(imaginary) plot tend tohave a linear shape, with little information on R_(p), hence it is notshown in the figure. Also shown in the figure is a simulated impedancedata with an R_(p)=36,000 ohm, C_(dl)=0.119 mF, and R_(conc)=2,400 ohm;these values were first estimated by fitting the experimental impedancedata to a (suppressed) semicircle. The close match between the simulatedand the experimental data suggests that the electrode/concrete interfacehas a polarization resistance, R_(p) of 36,000 ohms. The correspondingcorrosion rate is seven mils/year.

Furthermore, the extremely large value of the capacitance (0.119mF/0.078 cm² or 1.526 mF/cm²) is indicative of chloride adsorbed on thesteel surface. Thus, C_(dl) value can be used to infer the presence ofchloride, a potential corrosive agent, on steel buried in concrete.Measurement of R_(p) provides the rate of corrosion, independent of thereagents (chloride, acid rain, mercury in rain or fresh water, carbondioxide, microbes and so on) that cause the corrosion.

Several books on corrosion, including D. C. Silverman, “PracticalCorrosion Prediction Using Electrochemical Techniques” in Uhlig'sCorrosion Handbook, Ed. R. W. Reive, Electrochemical Society Series,Second Edition, John Wiley & Sons, NY, 2000, p. 1179, and Peabody'sControl of Pipeline Corrosion, Second Edition, Ed. R. L. Bianchetti.NACE International, TX, 2001, p. 307, describe the procedure to convertR_(p) to corrosion rate in units of mils/year. Typically, the conversionis made in three steps. First R_(p) is converted to corrosion current,I_(cor) using the formulaI _(cor)=(1/2.303R _(p))(b _(c) b _(a)/(b _(c) +b _(a))),where b_(c) and b_(a) are the cathodic and anodic Tafel slopes, whichfor steel in concrete are assumed to be 120 mV for b_(c) and 60 mV forb_(a). Next, I_(cor) is converted to weight loss, w in grams/cm²/s as(w/at)=(I _(cor) tMW)/nF,where MW is the molecular weight of iron (55.84), a is the area of theelectrode in square centimeter, t is the time, n is the number ofequivalence (2, for iron), and F is the Faraday constant (96,480 C).Taking into account the density, ρ of the metal, (7.86 g/cm³ for iron),the corrosion rate is expressed in conventional terms asCorrosion rate=(w/ρat)×1.242×10¹⁰ mils/year

The foregoing is considered as illustrative of the principles of theinvention. Accordingly all suitable modifications and equivalents may beresorted to, falling within the scope of the invention considered inlight of the appended claims.

1. An embeddable system for detecting and measuring corrosion in astructure susceptible to corrosion, said system including a plurality ofembeddable corrosion rate meters (ECRM) for collecting corrosionmeasurements data and at least one computing device for analyzing saidcorrosion measurements, said system comprising: at least one workingelectrode evenly separated from a counter electrode, wherein aseparation distance between said at least one working electrode and saidcounter electrode determines an electrolyte medium resistance, saidelectrolyte medium resistance is less than or equal to a polarizationresistance; a signal generator for generating a current source, saidcurrent source is connected to a plurality of resistances for creating aplurality of current amplitudes; a first selector for applying currentthrough each of said plurality of resistances to said at least oneworking electrode and said counter electrode, wherein said current isapplied via a galvanostat; an external reader-head with a data link andpower link connected to said computing device for powering said ECRM andtransferring corrosion measurements data via said data link; and aprogrammable electronic chip having a voltage output, wherein said chipis programmed to include a voltage-time signal, said voltage-time signalincluding a plurality of sine waves; and said galvanostat for receivingand converting said voltage output into a current-time perturbationsignal.
 2. The system of claim 1, wherein said ECRM is between 1 and 5centimeters in diameter and between 0.2 and 1 centimeters in height. 3.The system of claim 1, wherein said counter electrode is separated fromsaid at least one working electrode by holder material.
 4. The system ofclaim 1, wherein said working electrode is made from the same materialas the structure being detected for corrosion.
 5. The system of claim 4,wherein the material is a metal selected from the group consisting ofiron, carbon steel, stainless steel, super alloy steel, copper, zinc,aluminum, titanium, and alloys and combinations thereof.
 6. The systemof claim 1 wherein the structure is a rebar, storage tank, chamber,duct, tube or composite material.
 7. The system of claim 1, wherein saidcounter electrode is made from a non-corroding inert material.
 8. Thesystem of claim 7, wherein the non-corroding inert material is selectedfrom the group consisting of titanium oxide and ruthenium oxide,graphite, dimensionally stable palladium-coated titanium, and steel. 9.The system of claim 1, further comprising: a second selector forselecting the duration of a current pulse; and, a voltmeter and A-Dconverter for measuring polarization of said working electrode, whereinsaid voltmeter has an input impedance greater than 10⁹ ohms.
 10. Thesystem of claim 1, wherein said corrosion measurements data is used forgraphing a plot of I_(j) vs. (V_(p))_(j), with open circuit voltage OCVas the origin and estimating a slope of the plot of I_(j) vs.(V_(p))_(j), wherein said slope provides the value of the polarizationresistance, R_(p), which is inversely proportional to the corrosionrate.
 11. The system of claim 1, wherein said corrosion measurementsdata is obtained by disconnecting said galvanostat from said workingelectrode and said counter electrode and measuring a voltage differencebetween said working electrode and said counter electrode.
 12. Thesystem of claim 1, further comprising a unique electronicradio-frequency ID for identification of said ECRM.
 13. In a systemincluding: an embeddable system for detecting and measuring corrosion ina structure susceptible to corrosion, said system including a pluralityof embeddable corrosion rate meters (ECRM) for collecting corrosionmeasurements data and at least one computing device for analyzing saidcorrosion measurements, said system comprising: at least one workingelectrode evenly separated from a counter electrode, wherein aseparation distance between said at least one working electrode and saidcounter electrode determines an electrolyte medium resistance, saidelectrolyte medium resistance is less than or equal to a polarizationresistance; a signal generator for generating a current source, saidcurrent source is connected to a plurality of resistances for creating aplurality of current amplitudes; a first selector for applying currentthrough each of said plurality of resistances to said at least oneworking electrode and said counter electrode, wherein said current isapplied via a galvanostat; and an external reader-head with a data linkand power link connected to said computing device for powering said ECRMand transferring corrosion measurements data via said data link, amethod of obtaining the corrosion measurements data, comprising thesteps of: a) disconnecting said galvanostat from said working electrodeand said counter electrode; and b) measuring a voltage differencebetween said working electrode and said counter electrode, wherein stepb) comprises: setting a variable j to 0, where j is an integer valuefrom 0 to n; i) incrementing j and setting a current pulse amplitude toI_(j), wherein amplitudes for current pulses are in the ±0.1 to ±10 μArange; ii) starting a 1 ms current pulse at pre-set amplitude andmeasuring said voltage difference between working electrode and saidcounter electrode, storing said difference as 1 ms closed circuitvoltage (CCV_(@1 ms)) between said working electrode and said counterelectrode for the 1 ms current pulse at set amplitude I_(j); iii)starting a 500 ms current pulse at pre-set amplitude and measuring saidvoltage difference between working electrode and said counter electrode,storing said difference 500 ms closed circuit voltage (CCV_(@500 ms))between said working electrode and said counter electrode for the 500 mscurrent pulse at set amplitude I_(j), wherein a difference betweenCCV_(@1 ms) and CCV_(@500 ms) provides (V_(p))_(j); repeating stepsi)–iii, for current amplitude values of I₂ through [I_(j)]I_(n), as wellas at −I₁, through [−I_(j)]−I_(n), and estimating the value of(V_(p))_(j) for each I_(j) value.